The achieving of a non-cooperative radar function for detecting aerial obstacles for aircraft, in particular for drones, is essential in order to allow the insertion of autopiloted aircraft into the non-segregated aerial domain. It is part of the obstacle detection and avoidance function known by the name “Sense and Avoid”.
Such a radar must have a very wide observation field, typically ±120° in azimuth and ±15° in elevation, and must be capable of scanning space in a very short time, having regard to the time required to engage an avoidance manoeuvre in the event of a risk of collision. These characteristics correspond approximately to a “human” pilot's environment observation capacity.
Under these conditions, it is beneficial to use an antenna or several antennas with a wide transmission field, and to simultaneously form, on reception, multiple beams in the illuminated domain.
However, one problem is then the detection of mobile objects in flight against a background of significant ground clutter, in particular when the antenna beam is relatively wide and the level of the sidelobes is significant. This problem becomes all the more complicated to deal with the lower the altitude of the aircraft.
There is therefore a need to define a radar capable of detecting aircraft in flight that risk colliding with the carrier, whatever their approach speeds. Moreover, the volume, weight and cost of such a radar ought also to be minimized.
Radar devices meeting this requirement do not currently exist. However, comparable functions exist, notably for warplanes, which have an air-to-air detection mode. These radars cover a more extensive distance domain than that required in the present application defined above, but they cover a markedly smaller angular domain with a longer renewal period.
These modes use a directional antenna, which scans the monitoring domain mechanically or electronically, which is incompatible with the present application, having regard to the amplitude of the domain to be monitored and the maximum refresh time, which is of the order of a second.
Detection is conventionally performed by detecting the Doppler effect, thereby making it possible to some extent to separate the targets from the ground clutter.
These modes are known by the names HRF, MRF and LRF, corresponding respectively to the modes of transmission with high, medium and low recurrence frequency. They are amply described in the literature.
In the HRF mode, the detection of approaching slow targets is limited by the quality of the sidelobes of the antenna and exacerbated by the large amount of distance aliasing in the target search domain. Processing operations for minimizing these impediments exist, but they are very complicated.
In the MRF mode, the detection of both slow and fast targets may be affected by the clutter seen by the antenna sidelobes. The processing is complex on account of the need to simultaneously manage the distance ambiguities and the speed ambiguities. As previously, processing methods exist but remain very unwieldy in terms of implementation.
In the LRF mode, the “look down” detection performance is low on account of the large amount of speed aliasing, in particular:                The ground clutter of the main lobe occupies the major part of the ambiguous speed domain;        There is also a risk of false alarms on ground vehicles on account of the speed ambiguities, which mix up slow and fast vehicles;        The Doppler ambiguities are complex to remove;        Finally, this type of mode requires a high peak power.        
In any event, the solutions implemented on warplanes cannot be applied directly to an autopiloted aeroplane, having regard to the amplitude of the angular domain to be explored and also for obvious cost reasons.
Moreover, the speed domain and range domain for the obstacle detection and avoidance application are different from those of the air-to-air modes for warplanes.